The world’s oceans were warmer in 2019 than at any time in recorded history, a new analysis confirms.
The past five years all set records for the highest average annual ocean temperatures, with last year continuing the trend upwards.
The authors of the report, published in Advances in Atmospheric Science today, said climate change is unequivocally to blame for the consistent ocean temperature increase.
“This measured ocean warming is irrefutable and is further proof of global warming,” lead author Lijing Cheng from the Chinese Academy of Sciences said.
“There are no reasonable alternatives aside from the human emissions of heat-trapping gases to explain this heating,” Professor Cheng said.
Compared to the 1981-2010 average, they calculated that the 2019 average ocean temperature had increased by 0.075 degrees Celsius.
While that may not sound like much, the amount of energy the ocean has absorbed to warm by that much is about 228 sextillion joules — the equivalent of around 3.6 billion Hiroshima bomb explosions.
The single year increase in stored energy between 2018 and 2019 was equivalent to around 394 million Hiroshima bombs.
“That is a huge amount of energy,” Professor Cheng said.
Warming oceans influencing Australian bushfires
Rising ocean temperatures are leading to reduced dissolved oxygen and sea level rise, and are already increasing extreme weather events including floods and bushfires, the researchers said.
“It is one of the key reasons why the Earth has experienced increasing catastrophic fires in the Amazon, California and Australia in 2019, extending into 2020 for Australia,” they state in the paper.
“It is important to note that ocean warming will continue even if the global mean surface air temperature can be stabilised at or below 2 degrees C.”
While warming oceans have a complex influence on global weather patterns, there’s a simple general rule according to John Abraham of the University of Minnesota.
In Australia, that means the current hot, dry period we are experiencing has been made even hotter and drier, according to co-author Kevin Trenberth from the National Centre for Atmospheric Research.
“In 2019 the Indian Ocean Dipole became prominent and created a favourable weather pattern across east Australia for drought, heat waves and wild fires,” Professor Trenberth said.
“Global warming — extra heat — helps dry out the vegetation and increases [fire] risk.”
Around 90 per cent of climate change energy is stored in our oceans, according to Professor Abraham.
“You cannot really measure global warming unless you measure ocean warming,” he said.
“I like to say ‘global warming is ocean warming’, since the oceans absorb so much heat and are huge, it makes ocean measurements less noisy than air temperatures.”
In other words, because ocean temperatures take much more energy to move than air, they don’t fluctuate as wildly year on year as surface temperatures do.
They therefore provide a much more stable indication of warming trends.
The past 10 years have been the hottest 10 years on record, according to the researchers.
They also estimate the rate of warming during the period between 1987-2019 was 450 per cent greater than the prior 30-year period from 1955 to 1986.
The warming has been particularly pronounced in the upper 2,000 metres of the ocean, with the top 300 metres accounting for about 41 per cent of the increase.
Although warming was distributed across the world’s oceans, it was more pronounced in the Atlantic and Southern Ocean, where several severe marine heatwaves have also been recorded in recent years.